So… from a rudimentary google search:
DPS Arms FAQs page: #8. Is there a difference between 5.56 NATO and .223 Rem? Yes, despite external dimensions being identical, 5.56 NATO ammunition is a higher pressure round than commercial .223. If your rifle is chambered in .223 only .223 should be used as using .556 NATO can create an unsafe high pressure condition. If your rifle is chambered in 5.56 NATO it is safe to fire both .223 and 5.56 NATO.
Although most of the rifles on their site are listed as .223 & 5.56 NATO, their bull barreled target AR is listed as being chambered in only .223: DPMS Sweet 16.
This follows my assumption from the YT video and NRA article I listed that the .223 only chamberings might have a potential accuracy advantage (leade differences).
Although I can’t vouch for the third party posts (therefore I won’t link them), if you search on other forums, there seems to be two different answers regarding Remington’s R-15 line. Some customers pasted a response from Remington that they are .223 Rem only, while others pasted Bushmaster’s response (who supposedly made the Remington R-15 barrels) saying that the chambering was a hybrid with the tighter 223 dimensions, but with a leade resembling that for the 5.56 to handle the increased pressure. Furthermore, it’s confusing because their website doesn’t have a 5.56 option on their rifle website pages.
Interestingly enough, when looking at bolt actions, CZ states that it uses CIP (instead of SAAMI), and that CIP uses the 5.56 pressures to proof both the .223 Rem and 5.56 rifles coming out of that side of the Atlantic.
Further adding to the confusion, the website Balistics Tools goes into the similarity of higher pressure 223 to 5.56, but states that “pressures in any gun are affected a great deal by the throat and leade geometry” and that “The worst case scenario would be a full power 5.56 load, with a long bullet, fired from a brand-new .223 chamber that was cut to tight dimensions, with very little freebore, fired on a hot day, from brass with a lot of neck tension. Such a tight chamber is common on accurized match bolt-action .223 rifles. In such worst case conditions, pressures above 70,000 PSI have been reported. This is close to the proof pressure for .223, so there is potential danger firing 5.56 from a gun with a .223 chamber.”
Side note: the 70,000 PSI in the above statement refers to CIP standards (different PSI measurement method), not SAAMI.
Therefore, in a semi automatic rifle (which can get hot quickly) I might err towards caution when considering firing 5.56 in a .223 Rem only chambered rifle.